Rotor assembly for an electric motor and electric motor with internal rotor

ABSTRACT

An electric motor having a stator arranged on a sleeve; and a rotor assembly having a rotor shaft and at least one permanent magnet arranged on the rotor shaft, the rotor shaft being rotatbly housed in the sleeve. An external dimension of the rotor assembly and an internal dimension of the stator are selected such that the rotor assembly is free to move in the axial direction while the rotor assembly is inserted into the stator. The rotor assembly is aligned within the stator in the magnetic center of the stator by self-centering magnetic forces.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage U.S. Patent Application and claimsall rights of priority to International PCT Application No.PCT/EP01/07796, filed Jul. 4, 2001 (pending), which in turn claimed allrights of priority to German Patent Application No. 100 34 302.3, filedJul. 14, 2000 (pending).

FIELD OF INVENTION

The present invention refers to a rotor assembly for an electric motorwhich has a rotor shaft and at least one permanent magnet arranged onthe rotor shaft, an internal-rotor electric motor employing such a rotorassembly, and a method for manufacturing such an internal-rotor electricmotor.

The invention relates to the field of brushless electric motors havingpermanent magnets and specifically to the field of d.c. motorsconfigured as so-called internal-rotor electric motors. Internal-rotorelectric motors comprise a rotor assembly which includes a rotor shaftand one or more permanent magnets arranged on the rotor shaft and whichis inserted in a stator unit including a stator body and field windings.

BACKGROUND OF THE INVENTION

A construction of such a motor is described in U.S. Pat. No. 5,970,600.The motor comprises a frame in which the stator, the rotor assembly andbearings for rotatably journaling the rotor assembly are contained. Thestator comprises a stator iron core and windings and it defines aninterior space into which the motor assembly can be inserted. In U.S.Pat. No. 5,970,600, the bearings for the rotor assembly are integratedin end caps of the motor frame whereby a compact structure can beobtained altogether.

A problem arising in the manufacture of such an internal-rotor electricmotor is that when the rotor assembly is inserted into the interiorspace of the stator, ferro-magnetic particles may be abraded from therotor magnet and may enter the working air gap which is essentiallydefined by the outer contour of the rotor magnet and the inner contourof the stator. It is possible and common practice to protect the entiremotor assembly against penetration of foreign particles and soils byproviding a housing after the assembly, like in U.S. Pat. No. 5,970,600,but during the assembly of the various motor components the interior ofthe motor, and in particular the working air gap, is not protectedagainst the penetration of such foreign particles.

Moreover, internal-rotor electric motors according to the prior art havethe disadvantage of a relative time-consuming assembly, because it ismainly a sequential assembly in which the stator and at least onebearing must be successively pre-assembled in the housing and then therotor assembly must be fitted into the stator and bearing. In general,the required concentricity of the components is produced only by acover-like flange in which also the second bearing is located in whichthe rotor shaft is rotatably journaled. This assembling step is mademuch more difficult by the magnetic forces which come from the rotormagnet and are radially and axially directed by interaction with thestator iron core so that a contactless concentric fitting of the rotoris not possible at all or is possible only with major engineering.

From German patent No. 32 37 196, a synchronous micro motor is knownwhich comprises a one-piece pot-like housing which encloses as aferromagnetically effective yoke the ironless field windings andincludes a rotor arranged within the field windings. The permanentmagnets of the rotor are arranged on a sleeve made of a magneticallyconductive material. The rotor is housed in a hermetically sealedhousing containing the bearings and the torque is delivered through apermanent-magnetic coupling one part of which being formed by thepermanent magnets of the rotor and the other part of which consists of apermanent-magnetic arrangement on a shaft outside the rotor frame. Thehermetic encapsulation of the rotor ensures that no foreign particlescan penetrate into the rotor; the bearings are self-lubricated and areprotected against environmental impacts. The whole of the motor isassembled by inserting the hermetically encapsulated rotor into thecylindrical hollow space of a pot-like sleeve open on one side which ismade of a plastic in which sleeve the ironless field windings areembedded. This sleeve is located in a casing which is also formed like apot and is made of a ferromagnetic material.

The German patent DE 32 37 196 is already an improvement on the priorart described above, as the assembly of the motor is simplified, theabrasion of ferromagnetic particles from the rotor assembly during theassembly being eliminated from the start due to the ironless fieldwindings which are in addition completely embedded in the plastic.

The arrangement of DE 32 37 196 has the disadvantage that motors havingironless field windings always run with a very low efficiency because ofthe large air gap. Therefore, they are mainly used as micro motors forhigh speeds, preferably in the field of dentistry. However, for thedelivery of larger torques, as is required for motor vehicleapplications, for example, they are completely unsuited.

Moreover, the internal-rotor motor of DE 32 37 196 has the disadvantagethat the hermetic encapsulation of the rotor assembly does not allow adirect, mechanical coupling of a load, but that the torque transmissionfrom the rotor to a shaft must be only in an indirect manner, e.g. bymeans of magnetic coupling, as is described in the patent specification.

U.S. Pat. No. 4,999,533 discloses a motor having an enclosed rotor unitwhich is inserted into the stator core by press fit. GB-A-2,186,635discloses a centrifugal pump having a rotor enclosed in a cylindricalcomponent, the cylindrical component being inserted into a stator core.A similar arrangement is known from U.S. Pat. No. 3,733,504. Thisdocument additionally discloses that the sleeve of the rotor assemblycan be fixed in the stator by means of cement. To align the cylindricalcomponent, the stator core comprises stop edges.

SUMMARY OF THE INVENTION

It is the object of the invention to provide a rotor assembly for anelectric motor as well as an internal-rotor electric motor which can beassembled with minor-engineering within a short time and which ensurethat no foreign particles penetrate into the interior of the motor, andin particular into the working air gap, during assembly and operation.

This object is accomplished by a rotor assembly having the features ofclaim 1 as well as by an internal-rotor electric motor according toclaim 12. The invention also provides a method for manufacturing aninternal-rotor electric motor.

The rotor assembly according to the invention has the advantage that theentire rotor assembly can be pre-assembled in the sleeve, thepre-assembly being performed in a clean room, for example, to ensurethat no foreign particles enter the rotor assembly. The rotor assemblyis preferably designed in such a way that the rotor shaft is led out ofthe sleeve at one end thereof, the sealing between the sleeve body andthe rotor shaft being performed through bearings in the end faces of thesleeve which rotatably journal the rotor shaft. This does not produce ahermetical sealing of the rotor assembly, but the sleeve is sufficientlysealed in its end faces by the bearings, which are in addition protectedby seals against the penetration of foreign particles, to avoid thepenetration of solid particles. One end face of the sleeve shouldcomprise an opening for leading out the rotor shaft, whereas theopposite end face may be completely closed as required.

The sleeve according to the invention avoids the above-described problemof the prior art that when the rotor assembly is inserted into thestator, ferromagnetic particles are abraded from the permanent magnet(s)and enter the air gap. The sleeve enclosing the rotor is preferably madeof plastic and the rotor as a whole is inserted as a pre-assembledassembly into the interior of the stator so that no problems with theabrasion of ferromagnetic particles can occur.

As the rotor can be indirectly fitted into the plastic sleeve throughthe bearings with a very small concentricity error and the plasticsleeve, in turn, can be fitted into the stator almost free frombacklash, a concentricity between rotor and stator which issubstantially improved on the prior art can be obtained. In addition, ametallic contact between the bearings and any frame components such asflange, cover and the like is eliminated from the start so that thetransmitted structure-borne noise can be substantially damped and,hence, also the acoustic emission can be reduced by up to 10 dBaccording to first studies performed by the applicant.

The rotor assembly enclosed in the plastic sleeve has the furtheradvantage that it self-centres in the axial direction by the magneticforces between the rotor assembly and the stator when it is insertedinto the stator so that no further measures must be taken, such as theprovision and adjustment of stops, to properly position the rotor withinthe stator. It only should be taken care to ensure that the rotorassembly can move within the stator sufficiently freely in the axialdirection so that no stop, closed end or the like prevents the rotorassembly from magnetically centring by itself.

The sleeve according to the invention to be used for the rotor assemblyconsists of a sleeve wall and preferably an additional stiffeningstructure on the outside of the sleeve wall. This ensures that thesleeve is, on the one hand, sufficiently stable and, on the other, canbe used even if the working air gap determined by the inner contour ofthe stator and the outer contour of the rotor is very small. The actualsleeve wall may be thinner than is necessary for the stability of thesleeve, because the stiffening structure gives additional strength tothe sleeve. If the air gap is sufficiently large, the sleeve wall mayalso be dimensioned without a stiffening structure in such a way that itis sufficiently stable. The stiffening structure is preferably formed asribs in such a way that the outer contour of the stiffening on theoutside of the sleeve is adapted to the inner contour of the stator.This results in an optimum utilization of the interior space of thestator and, in addition, in the advantage that the rotor assembly can beinserted into the stator free from backlash and in a torsion-resistantmanner, because the outer contour of the stiffening structure and theinner contour of the stator engage.

The rib structure advantageously comprises in the longitudinal directionof the sleeve or diagonally thereto ribs extending along the outside ofthe sleeve so that the rotor assembly can be inserted into the stator byguiding it rectilinearly or along a helical line through the innercontour of the stator.

For easy assembly, bearings for supporting the rotor shaft arepre-mounted thereon so that the rotor can be inserted into the sleevealong with the bearings.

According to one embodiment, the sleeve comprises a substantiallycylindrical encapsulation section and a flange section which can beengaged with each other. That is, the encapsulation section and theflange section are slid over the rotor shaft from opposite ends and formtogether a closed frame around the rotor.

The sleeve is adapted to the shape of the rotor and is generallycylindrical. As described below, the sleeve may have a two-piece or, forexample, a three-piece design having a cylindrical central portion andtwo end sections.

The bearings for supporting the shaft are preferably located in the endfaces of the sleeve in such a way that they bear against stops so thatthe rotor assembly, after the encapsulation and flange sections havebeen assembled, is enclosed in the sleeve in a sealed manner.

Further, the invention provides an internal-rotor electric motor havinga rotor assembly of the type described above and a stator, the rotorassembly being inserted in the stator and the sleeve comprising a wallthickness equal to or smaller than a working air gap between an outerdiameter of the rotor and an inner diameter of the stator.

Preferably, the outer contour of the stiffening structure on the outsideof the sleeve is adapted to the inner contour of the stator, the ribs ofthe stiffening contour being located between the stator poles.

Alternatively or additionally, the stator sheets of the stator may beprovided with inner grooves on the stator poles which can be engagedwith additional stiffening ribs on the outside of the sleeve.

Preferably, the rotor assembly is inserted into the stator free frombacklash in such a way that the rotor centres by itself within thestator core in the axial direction.

Thus the invention provides a rotor assembly for an electric motor andan internal-rotor electric motor in which the rotor is pre-assembled ina sleeve along with the bearings and this rotor assembly is insertedinto the stator. The rotor shaft is led out of the sleeve, the sleevewall along with the bearings and the seals thereof ensuring a sufficientsealing of the rotor assembly. In addition, the stiffening structureallows the sleeve to be formed in such a way that it has a very thinwall so that this rotor assembly is suited even for a very small workingair gap between stator and rotor. The stiffening structure may bedesigned in such a way that it is adapted to the inner contour of thestator and, for example, engages with the pole gaps of the stator. Theconstruction according to the invention yields a very compact, andsimple as well, rotor assembly which can be completely pre-assembled ina clean room, for example, and is then inserted as a whole into thestator. The provision of the sleeve can prevent ferromagnetic materialfrom being abraded and from penetrating into the working air gap whenthe rotor assembly is inserted into the stator. The encapsulated rotorassembly also prevents the penetration of other foreign particles.Moreover, the encapsulation of the rotor assembly having a sleevepreferably made of plastic leads to the effect that the rotor centres byitself in the axial direction when it is inserted into the stator andthat no structure-borne noise is transmitted from the rotor to thestator or the motor frame as appropriate.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained below by way of preferred embodiments thereofwith reference to the drawings, wherein:

FIG. 1 shows a cross-sectional view of a rotor assembly according to theinvention;

FIG. 2 shows a cross-sectional view of an encapsulation section;

FIG. 3 shows a top view of the encapsulation section of FIG. 2;

FIG. 4 shows a cross-sectional view of the flange section of the sleeveaccording to the invention;

FIG. 5 shows a bottom view of the flange section of FIG. 4;

FIG. 6 shows a top view of a stator sheet; and

FIG. 7 shows a cross-sectional view of an internal-rotor electric motoraccording to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a cross-sectional view of a preferred embodiment of a rotorassembly according to the invention. The rotor assembly according to theinvention comprises a rotor shaft 10 carrying a yoke ring 12 made of asoft magnetic material such as iron. A preferably annular permanentmagnet 14 is attached on the yoke ring 12. The shaft 10 is rotatablyjournaled in bearings 16, 18, wherein the bearings 16, 18 may be formedas antifriction or friction bearings and specifically as rollerbearings. The rotor which is formed here by the rotor shaft 10, the yokering 12 and the permanent magnet 14 is enclosed in a sleeve 20comprising an encapsulation section 22 and a flange section 24 whichwill be explained in detail with reference to the FIGS. 2 to 5.

The bearings 16, 18 may be pre-mounted on the rotor shaft 10 and areplaced in the end faces of the sleeve sections 22, 24 and are pressed inthem and/or adhered to them or are held in another suitable way. In theillustrated embodiment, an annular spring element, e.g. an ondularwasher 26, is provided in the end face of the flange section 24 adjacentto the bearing 18 for the compensation of the axial sum tolerances.Through this ondular washer, the two bearings 16, 18 can be additionallybraced against each other free from backlash.

The rotor assembly according to the invention is shown with its basicelements in FIG. 1, wherein the specific dimensioning and particulararrangement of the elements may vary. Specifically, roller bearings 16,18 are provided in FIG. 1, wherein the rotor may alternatively berotatably journaled in any other suitable way such as by frictionbearings, hydraulic bearings, air bearings, etc. Also, the rotorassembly may farther comprise other elements then those illustrated.Between the outer circumference of the permanent magnet 14 and theinside of the sleeve 20, an air gap as small as possible is providedwhich allows the relative movement between the fixed sleeve and therotating rotor. Depending on the requirements of the rotor assembly andelectric motor, many other modifications of the embodiment shown willbecome apparent to a person skilled in the art.

The sleeve 20 is preferably made of plastic by injection moulding orextrusion moulding. A particularly suitable plastic is LPC (liquidcrystal polymer). Other materials include polyacetal, polyoxymethylene(POM), polysulphone (PSU), polycarbonate (PC), polyphenylene sulphide(PPS), polyamide imide (PAi), polyether ether ketone (PEEK), polyethersulphone (PES), polyether imide (PEi).

In the embodiment shown, the sleeve 20 is composed of a cup-likeencapsulation section 22, which is shown in detail in FIGS. 2 and 3, anda cover-like flange section 24, which is shown in detail in FIGS. 4 and5. This is only one embodiment of the invention, wherein the sleeve 20may alternatively be formed of two substantially similar engaging cupsections or of three parts or may be formed in any other suitable way.

The encapsulation section 22 shown in detail in FIGS. 2 and 3 consistsof an end wall 32 and an adjacent sleeve portion 30 having a relativethin inner wall. On the outside of the sleeve portion 30, longitudinalribs 34 are formed, 18 longitudinal ribs 34 being provided in theembodiment shown. Of course, the invention is not limited to aparticular number of ribs as long as a sufficient number of ribs isprovided to give the sleeve 20 the required stiffness.

In the embodiment shown, the longitudinal ribs 34 extend substantiallyover the entire length of the cylindrical sleeve 20 and are alignedparallel to the longitudinal axis thereof. In other embodiments, theribs might extend diagonally, for example, the number and design of theribs being adapted to the inner contour of the stator, as will becomeapparent with reference to FIG. 6.

FIG. 6 shows a single stator sheet 50 having stator poles 52 and polegaps 58. In the embodiment shown, the longitudinal ribs 34 are formedand arranged on the outside of the encapsulation section 22 of thesleeve 20 in such a way that these longitudinal ribs 34 engage with therecesses of the pole gaps 58 when the rotor assembly is inserted intothe stator.

In another embodiment, ribs may be additionally or alternatively formedand arranged in such a way that they engage with grooves (not shown)arranged on the radially inwardly directed end faces of the statorpoles.

It is not crucial to the invention that the number of ribs coincideswith the number of pole gaps, but they may rather be in any proportionto each other.

If the ribs, as in the embodiment shown, are formed as longitudinal ribs34 running parallel to the longitudinal axis of the sleeve 20, the wholeof the rotor assembly can be rectilinearly inserted into the stator. Ifthey are, for example, diagonally arranged on the sleeve, the rotorassembly would have to be “screwed into” an appropriate stator havingangular pole gaps, the stator in this case being constructed of statorsheets twisted relative to each other by a particular angle.

Referring again to FIGS. 2 to 5, the flange section 24 of the sleeve 20is formed with a relative short side wall 40 and an end wall 42 in sucha way that the flange section 24 can be inserted into the encapsulationsection 22. Preferably, the sleeve sections 22, 24 are joined byadhesive bonding, welding or in another suitable way.

In each of the two end walls 32, 42 of the sleeve sections 22, 24, onefit 38 and 48, respectively, is formed, into which the roller bearings18, 16 or any other suitable bearings for supporting the shaft 10 can befit.

One embodiment of the internal-rotor electric motor according to apreferred embodiment is schematically illustrated in FIG. 7. The sameelements or elements similar to those in FIGS. 1 to 6 are denoted by thesame reference symbols and are not explained here again.

FIG. 7 shows the rotor assembly according to the invention explainedwith reference to FIG. 1 which is inserted in the interior of a stator56. The stator 56 is schematically illustrated in FIG. 7 by a stack ofsheets constructed of individual stator sheets 50 and by a winding 60. Abroken line 42 indicates the engagement of the longitudinal ribs 34 inthe pole gaps 58 formed between the stator poles 52.

The internal-rotor electric motor according to the invention can beeasily manufactured by first pre-assembling the rotor assemblypreferably in a clean room. To that end, the iron yoke ring 12 and thepermanent magnet 14 are attached to the rotor shaft 10 and the bearings16, 18 are pre-mounted on the rotor shaft 10. The two sleeve sections22, 24 are slid over the rotor from opposite ends of the shaft 10 andare joined with each other and are preferably additionally bonded orglued to each other. The rotor bearings 16, 18 then fit tightly in theend faces 32, 42 of the sleeve sections 22, 24 and are braced by theondular washer 26.

Before the completely pre-assembled assembly is inserted into theinterior of the stator 56, in which the assembly is free to move in theaxial direction, and is aligned by “self-centring magnetic forces” intothe magnetic centre, a preferably liquid plastic may be applied to theoutside of the sleeve for permanently connecting the assembly to thestator.

In the embodiment shown, the two ends of the stator ring are open so asnot to constrain the axial movement of the rotor assembly. Otherconstructions of the stator will be apparent to a person skilled in theart, the invention offering the major advantage that no stops or othermeans for centring the rotor within the stator are needed.

After the rotor assembly has been inserted into the stator, the adhesivefills the still existing spaces at least partially and ensures aftercuring a tight, stationary fit of the rotor within the stator.

The complete encapsulation of the rotor and the enclosing of all moving,rotating parts in a sleeve which is fixed relative to the stator and amotor frame as appropriate drastically reduce the transmission ofstructure-borne noise and hence the acoustic emission of the whole ofthe motor, as was mentioned above.

The internal-rotor motor constructed in this way may now be moulded inor potted or may be inserted into an encasing frame or may be providedwith a fixing flange arranged on the end face, without impairing thefunction in any way.

The stator 56 does not need to be particularly adapted to the rotorassembly according to the invention. The connections between theindividual components such as shaft 10, yoke ring 12 and permanentmagnet 14 or between encapsulation sections 22, 24 and bearings 16, 18may be made by injection, adhesive bonding, welding, latching or in anyother suitable way. The sleeve 20 and the stator 26 may be agglutinatedafter joining. To avoid an abrasion of plastic material of the sleeve 20when the rotor is inserted into the sleeve, the edges of the permanentmagnets 14 may be rounded or chamfered. Numerous further alterations andmodifications of the invention will become apparent to a person skilledin the art.

The wall thickness of the sleeve portion 30 is selected such that itsubstantially fills the air gap between the permanent magnets 14 of therotor and the inside of the stator 56, wherein, due to the ribstiffening, the inner wall of the sleeve portion 30 may alternatively bethinner than this air gap or the encapsulated rotor assembly accordingto the invention may be used even if the working air gap is very small.The ribs 34 are, as was explained above, formed in such a way that theyare adapted to the inner contour of the stator and that they engage withthe inner contour and that they give additional stiffness to the wholeof the sleeve.

In addition to the embodiment shown, it may be provided to encapsulatethe stator 56 as well and to mold it in plastic, for example.

The features disclosed in the above description, in the claims and thedrawings may be, both individually and in any selection and combination,important to the realization of the invention in the various embodimentsthereof.

List of Reference Symbols

-   -   10 Rotor shaft    -   12 Yoke ring    -   14 Permanent magnet    -   16 Roller bearing    -   18 Roller bearing    -   20 Sleeve (cylindrical)    -   22 Encapsulation section    -   24 Flange section    -   26 Ondular ring    -   30 Sleeve portion    -   32 End wall    -   34 Longitudinal ribs    -   38 Fit    -   40 Side wall    -   42 End wall    -   48 Fit    -   50 Stator sheet    -   52 Stator pole    -   56 Stator    -   58 Pole gaps    -   60 Winding

1. An electric motor comprising: a stator having a sleeve, said sleevehaving a cylindrical portion and a flange portion, said cylindricalportion and said flange portion of the sleeve being connected; a rotorassembly having a rotor shaft and at least one permanent magnet arrangedon the rotor shaft, the rotor shaft being rotatably housed in thecylindrical portion and the flange portion of the sleeve; and a firstbearing and a second bearing supporting said rotor shaft for rotationwithin said sleeve; wherein said first bearing is integrated into thecylindrical portion of said sleeve, wherein said second bearing isintegrated into the flange portion of said sleeve, wherein an externaldimension of the rotor assembly and an internal dimension of the statorare selected such that the rotor assembly is free to move in the axialdirection while the rotor assembly is inserted into the stator, andwherein the rotor assembly is aligned within the stator in the magneticcenter of the stator by self-centering magnetic forces.
 2. The electricmotor of claim 1, wherein the sleeve further comprises a stiffeningstructure on the outside of the sleeve.
 3. The electric motor of claim1, wherein the sleeve further comprises a rib structure on the outsideof the sleeve.
 4. The electric motor of claim 3, wherein the ribstructure comprises a plurality of ribs extending in the longitudinaldirection of the sleeve or diagonally thereto along the outside of thesleeve.
 5. The electric motor of claim 1, wherein the rotor shaftextends outside of the flange portion of the sleeve at a first end ofthe sleeve.
 6. The electric motor of claim 5, wherein a second end ofthe sleeve is substantially closed by said cylindrical portion.
 7. Theelectric motor of claim 1, wherein said first bearing and said secondbearing are arranged at end faces of the sleeve enclosing the rotorassembly in the sleeve in a sealed manner.
 8. The electric motor ofclaim 1, wherein the motor is an internal-rotor motor.
 9. The electricmotor of claim 1, wherein the stator further comprises stator sheetshaving stator poles pointing towards the interior of the stator.
 10. Theelectric motor of claim 1, wherein a working air gap is formed betweenan outer diameter of the rotor and an inner diameter of the stator, andwherein the sleeve further comprises a wall having a thickness which isequal to or less than the working air gap.
 11. The electric motor ofclaim 2, wherein an outer contour of the stiffening structure on theoutside of the sleeve is adapted to an inner contour of the stator. 12.The electric motor of claim 11, wherein the stiffening structure furthercomprises ribs located in pole gaps provided between the stator poles.13. The electric motor of claim 1, wherein the rotor assembly isinserted into the stator free from backlash.
 14. The electric motor ofclaim 1, wherein the motor is potted.